1. Field of the Invention
The invention relates to a method for synchronising a receiver with a signal, said signal having a frequency which is not accurately known in advance.
2. Description of the Prior Art
In such radio communications systems that have several data transmission frequencies and variable uses with respect to area and/or time, the receiver must, prior to beginning the reception properly, find the desired signal and synchronise its operation in order to interpret the content of the signal. Finding the signal means that the receiver is tuned to exactly that frequency where the signal is located. In the synchronisation process the receiver must find out where each separate symbol pertaining to the signal begins, and how the frequency and phase are changed during reception.
The present application pays special attention to the I-CO Global Communications satellite telephone system, which is based on ten communications satellites with a so-called medium-high orbit (roughly 10,000 km). The satellites orbit the earth at regular intervals on two mutually perpendicular orbits with an inclination of 45.degree.. Each satellite comprises an antenna arrangement with a power pattern of 121 narrow radiation lobes, which together cover the coverage area of said satellite on earth. The coverage area means the whole area Erom which the satellite is seen more than 10 degrees above the horizon. The operational range of the system is roughly 2 GHz, and it utilises TDMA, Time Division Multiple Access.
As a concept, the system defines a so-called CCS carrier (Common Channel Signalling), which means a given carrier frequency reserved for signal acquisition, synchronisation and distribution of general communications information. Globally there are reserved 120 frequencies for CCS carriers, and these frequencies are further grouped into regional and local frequencies. When a certain satellite moves on its orbit, its coverage area moves along the surface of the earth. The satellite changes the transmitted CCS frequencies in between the separate radiation lobes, so that in a given geographic area, there are always received the same frequencies. A receiver located on earth or near the surface of the earth stores the eight location-connected local CCS frequencies to a non-volatile memory; consequently, when it is switched off and back on, it searches a signal among said eight frequencies. If a signal is not found, the receiver next surveys the 40 regional frequencies, and if there still is no signal, finally all 120 global frequencies.
According to FIG. 1, a transmission with each CCS frequency consists of several multiframes 1, which are divided into 25 slots 2. Each slot includes 120 symbols 3. According to current definitions, the symbol rate in the system is 18,000 symbols per second, but it may be increased to 36,000 symbols per second in the future. The first slot in the frame comprises a BCCH (Broadcast Control Channel) burst 4, which is BPSK (Binary Phase Shift Keying) modulated and contains, among others, communications data and a 32 symbols long reference sequence 5, which is important for synchronisation. The location and form of the reference sequence inside the BCCH burst will be essentially fixed and known. Two successive slots contain a FCH (Frequency Channel) burst 6, which is transmitted with a somewhat lower level than the BCCH burst and consists of pure sinus wave at the frequency of said CCS channel; the purpose of said FCH burst 6 is to aid the synchronisation of the receiver. Other slots in the CCS carrier are empty.
For successful reception, the receiver must, after being switched on, first find the desired signal. General criteria for the signal to be found is that the timing error in the reception is .+-.1/2 symbols at the most, and that the frequency error is no more than a few percentages of the symbol rate. The nearer to zero these two error factors are, the smaller the probability that bit errors happen in the reception, and the less the reception is sensitive to the deterioration of the S/N ratio. An advantageous method for fulfilling these criteria is introduced in U.S. patent application "Signal acquisition in a satellite telephone system" filed simultaneously with the present application and by the same applicant under Ser. No. 08/859,500.
Problem to be Solved:
After finding the signal, however, the problem is how to further diminish the timing and frequency errors from the above described coarse values, and how synchronisation is maintained in a system where the transmission or link stations (satellites) and receivers (mobile terminals) move with respect to each other at varying speeds causing doppler shift of the reception frequency and phase error. According to current usage experiences, a moving terminal of a data transmission system, such as a mobile telephone, is most of the time in idle mode, where power is switched on but the user is not in active communication with anybody. Low electricity consumption is an important factor in mobile terminals, and therefore the receiver should be switched on as rarely as possible during the idle mode which condition is, however, contradictory to the aim to accurately maintain the synchronisation. Other factors affecting the on-state periods of the receiver are various standards and definitions pertaining to individual data transmission systems and dealing with the reception of a call or a system message by the receiver without immoderate delay. Moreover, it is advantageous for reducing the complexity of the receiver and for cutting production costs if the synchronisation does not require a very high calculation capacity of the receiver device.